This invention relates to an oxygen sensor used in detecting the oxygen concentration in a gas such as the exhaust gas from an internal combustion engine of automotive vehicles. More particularly, it relates to a solid electrolyte consisting essentially of sintered zirconia, which has a good stability of crystal structure and a good oxygen ion conductivity.
Among the solid electrolytes of this sort for use in an oxygen sensor, there has been known a sintered body of partially stabilized zirconia which is prepared by mixing zirconia with a stabilizer such as yttria, calcia, or ytterbia followed by sintering at high temperatures and has a mixed structure of a monoclinic and/or tetragonal phase and a cubic phase, as disclosed in U.S. Pat. No. 4,219,359 (Miwa et al.).
This known sintered body of partially stabilized zirconia has a high strength and excellent thermal shock resistance, because it has a fine structure and is a composite material.
However, it has been found that the composite material causes the structure of the partially stabilized zirconia to be unstable. That is, the proportions of the tetragonal phase content and the monoclinic phase content in said sintered body are variable at the temperatures of using a solid electrolyte for an oxygen sensor, i.e., about -40.degree. to 900.degree. C. The repeated changes in temperature within the range of about -40.degree. to 900.degree. C., or the continuous use of a sensor at a lower temperature range, e.g., about 200.degree. to 300.degree. C. cause the phase transition from the tetragonal phase to the monoclinic phase to take place. Furthermore, a stable cubic phase is partially transformed to the monoclinic phase.
Since the phase transition to the monoclinic phase is accompanied by volume expansion, it causes the structure of the sintered body to be breakable. In addition, since the monoclinic phase content is increased by the phase transition, the sintered body becomes poor in an oxygen ion conductivity. Thus, the service temperature of an oxygen sensor is biased to a higher, narrowly restricted range.
Therefore, it is an important problem how to reduce the monoclinic phase content in a solid electrolyte made of the partially stabilized zirconia, i.e., to prevent the monoclinic phase content from being increased by the phase transition during service.
Thus, the inventors have found that when the partially stabilized sintered zirconia was finely ground, there may be detected a monoclinic phase.